This invention relates to the combustion of fuel to produce heat. In one of its aspects this invention relates to the controlled combustion of fuel. In another of its aspects this invention relates to supplying to a combustion chamber an amount of fuel ratioed to an amount of combustion air supplied. In still another of its aspects this invention relates to control of fuel flow during a period of shifting from a primary means of supplying combustion air to a secondary means for supplying combustion air.
This invention has arisen from a specific situation in which hot exhaust gas from a combustion gas turbine is used as a source of heat and oxygen for burning supplemental fuel for a boiler or a heater. The invention is applicable, however, in any system in which combustion air is supplied from a primary air mover that has a backup, secondary air mover that is used in time of emergency shutdown of the primary means for supplying combustion air. Most combustion gas turbines supplying exhaust gas to waste heat boilers, for instance, have a fresh air blower that is started and supplies combustion air when the gas turbine is shut down or tripped off.
The control of fuel and air supply in these combustion systems usually incorporates a conventional "lead-lag" fuel and air control system which, among other things, will detect a condition of insufficient combustion air and match it with a fuel rate that is ratioed to be safe regardless of the demand. Past experience has proven, however, that upon emergency shutdown of the primary means for supplying combustion air there is usually such rapid decay of air supply that the control limiting the fuel is unable to keep pace, causing the heating system to go through a dangerous period of excess fuel. Up to now it has been common practice to run the secondary means for supplying combustion air at all times so that it will be up to speed at any time it might be needed. Avoiding the use of energy required for running the standby system when it is not needed would, of course, improve the economics of the combustion system, but, more importantly, would eliminate a useless expenditure of energy.
It is, therefore, an object of this invention to provide a safe and economical method for controlling the flow of fuel to a combustion chamber during the shifting of operation from a primary means for supplying combustion air to a secondary means for supplying combustion air. It is another object of this invention to provide apparatus for controlling the flow of fuel to a combustion chamber during the shifting from a primary means of supplying combustion air to a secondary means of supplying combustion air.
Other aspects, objects, and the various advantages of this invention will become apparent upon study of this specification, the drawings, and the appended claims.